By default, when you sync an iPhone or iPad, connect a camera, or insert an SD Card into your Mac, OS X opens iPhoto, asking you if you want to import photos. I had this setting turned off in Mavericks, but it seems to have been turned on again in Yosemite. Yet when you look in iPhoto, there is no such setting. Tricky of Apple to have hidden it in another app…
Connect your iOS device to your Mac. Open Image Capture, which is in your Applications folder. Click on the iOS device, then you’ll see, below the sidebar, an option allowing you to choose what happens when you connect the device.
(Yes, I have lots of photos of Titus the Cat.)
Click the menu that says iPhoto; you have several options, such as Image Capture, Preview, or, perhaps, other apps. But you can also choose No Application. Select that, and you’re good to go. Personally, I use Dropbox’s photo import feature, then go through my photos in the Dropbox folder from time to time and decide what I want to keep.
If you don’t see the menu below the sidebar, click the little widget at the bottom-left of the window; that will display the menu.
Note: if you’ve turned on Photos in the Cloud in your iOS device settings, you won’t see this option, since your iOS device no longer lets you add photos to iPhoto when it’s connected. And you can’t delete photos from the device using Image Capture either.
Update: a commenter pointed out that there’s a setting in iPhoto’s general preferences. I missed this – in part because I’m pretty sure it wasn’t there in Mavericks – but that setting is global, and affects what happens when you connect any camera or iOS device. In Image Capture, you can adjust this setting individually for each device, so the Image Capture solution is more flexible.